As for commonly used direct-current power supply, there are a switching regulator and a series regulator. Switching regulators are used for many apparatuses since they provide high efficiency. With the switching regulator, however, there is a considerable drop in its efficiency when supplying power to a small load which consumes little current due to the switching regulator having a large ripple in output power and noise during operation and consuming a relatively large amount of power inside. Furthermore, the switching regulator has a disadvantage of being liable of considerably damaging the load due to the switching regulator having a relatively slow rise of power and a relatively slow response time for input power change and load change.
Therefore, conventionally, a soft start circuit is provided for preventing generation of overshoot noise by delaying the output voltage rise when turning on the power supply (For example, Japanese Laid-Open Patent Application Nos. 2000-102243 and 2001-128445).
FIG. 5 is a circuit diagram of a conventional example of a switching regulator provided with a soft start circuit. It is to be noted that although FIG. 5 shows a switching regulator of a type which lowers voltage of input power, a switching regulator of a type which raises voltage of input power may be used alternatively.
The switching regulator illustrated in FIG. 5 includes a switching transistor Ma for executing output control of input voltage Vin, an inductor La along with a condenser Ca for energy conversion, a transistor Mb for synchronous rectification (synchronous rectification transistor), a PWM control circuit for executing switching control between the synchronous rectification transistor Mb and the switching transistor Ma, and resistances Ra and Rb for output voltage detection. Furthermore, the switching regulator 100 includes a reference voltage generation circuit 102 for generating and outputting reference voltage Vref, an amplifier AMP for executing operation of the PWM control circuit 101 in accordance with voltage difference between the reference voltage Vref and divided voltage of output voltage Vout being divided at the resistances Ra and Rb, a time constant circuit including a resistance Rc and a condenser Cb for applying the reference voltage Vref, in a gradually rising manner, to the amplifier AMP when turning on the power, and a switch SW1.
Here, only the soft start circuit is described below.
The switch 1 is switched on at the same time when power is turned on, and thus the condenser Cb is charged with the reference voltage Vref via the resistance Rc. Accordingly, voltage Va at a noninverting input terminal rises gradually in a manner shown in FIG. 6. Since the output voltage Vout of the switching regulator 100 is proportional to the reference voltage Vref, the output voltage Vout also rises gradually in a manner shown in FIG. 6. This prevents noise from being generated by overshoot.
However, with such method of gradually raising the output voltage Vout to a predetermined voltage by gradually raising the reference voltage, a certain amount of time is needed for the output voltage Vout to reach the predetermined voltage. Accordingly, after the power is turned on, a considerable amount of time is required until commencing operation of an apparatus.